Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Referring to FIG. 1, typically a turbofan gas turbine engine 1010 will comprise an axial fan 1012 driven by an engine core 1015. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The turbofan gas turbine engine 1010 is generally provided with outlet guide vanes (OGVs) 1032 downstream of the fan 1012 for straightening flow from the fan. Typically one or two bifurcations 1038 are provided downstream of the OGVs between the inner and outer walls of the bypass duct. The bifurcations include aerodynamically shaped fairings around the pylon structure for the purpose of encasing and directing electrical connections. Usually one bifurcation is positioned between the engine core 1015 and the outer casing at a position near a pylon 1008 that mounts the engine to the wing of an aircraft. If two bifurcations are provided, the second bifurcation is generally positioned diametrically opposite the bifurcation shown in FIG. 1.
However, the bifurcation provides a substantial and undesirable blockage to the flow downstream of the OGVs. This means that part of the flow from the OGVs stagnates at the bifurcation leading edge. The stagnated flow reaches a high static pressure equal to the incoming total pressure, also called stagnation pressure. The remaining flow from the OGVs flows around the bifurcation which results in acceleration, hence a reduction in pressure. The resulting high leading edge stagnation pressure and the low pressure around the area of high pressure bring a significant peak-to-peak variation in the static pressure field seen from the OGV trailing-edge.
This problem is illustrated in FIGS. 2A and 2B. Referring to FIG. 2A, the static pressure field is indicated at 1054 and the resulting force on the fan is indicated by arrow F referred to as “forcing” in the art. Forcing on the fan blades causes undesirable fan blade vibration. Referring to FIG. 2B, the region between the bifurcation 1038 and the OGVs 1032 (both shown in FIG. 2A) is illustrated, the region being defined by an outer casing 1036 and an inner casing 1034. As can be seen in FIG. 2B, an area of high static pressure 1052 is formed in the region of the two bifurcations, and a region of low pressure 1050 is formed between the two bifurcations.
One method of addressing this problem is to alter the stagger angle and camber of the OGVs. When the OGVs are angled and cambered to deliver a flow guided around but close to the bifurcation, the OGVs mask none of the static pressure field. This arrangement of OGVs is often referred to as “transparent”. This is a poor case from a rotor forcing view point because the rotor sees the entire static pressure field variation. The static pressure field can be masked, often referred to as an “opaque” arrangement, by altering the camber and the stagger angle of the OGVs to deliver a flow direction that is angled further away from the bifurcation than the “transparent” arrangement. The “opaque” arrangement is better than the “transparent” arrangement from a forcing view point, but suffers from increased aerodynamic losses. Generally, gas turbine engines of the prior art seek a balance between the reduced forcing of the “opaque” arrangement and the improved aerodynamic performance of the “transparent” arrangement.